CEUSTACEA OF NEW ZEALAND. 



249 



Some of the other rivers sliow rather more gradual falls than those quoted in the 

 tahle ahove ; thus the River Selwyn from its entrance into the plains to its mouth at 

 Lake Ellesmere has a total fall of 790 feet in a distance of 29 miles, or an average of 

 27 1 feet per mile. 



With regard to the origin of the plains Sir Julius von Haast says : — " All the levels, 

 surveys, engineering works, together with Avell-sinking, have amply confirmed my views 

 that the Canterbury Plains are of fluviatile origin, that, with the exception of some 

 morainic accumulation in the upper portion and the drift sands round Banks 

 Peninsula, and tlie partial laciistrine deposits filling the former extension of Lake 

 Ellesmere, the whole of the plains were formed by huge rivers issuing from tlie frontal 

 end of gigantic glaciers" [53, p. 396]. Consequently the Canterbury Plains consist of 

 river shingle and sand, cemented more or less by a ferruginous matrix ; this has been 

 proved by the sinking of wells in many directions on the Plains, some of them to con- 

 siderable depths. In a well between the Ashburton and Rakaia, " where a supply of 

 water was reached at a depth of 220 feet, the shingle at the bottom became much 

 cleaner and incoherent, resembling the small shingle in the Rakaia river-bed " 

 [53, p. 473]. 



All over the plains water is found at a greater or less depth below the surface, the depth 

 varying according to the locality, the dryness of the season, the condition of neigh- 

 bouring rivers, &c. In a great many places this water is found within about 30 feet of 

 the surface, and can hence be brought up by suction-pumps ; in some cases, however, as 

 in the one quoted above, water is not found until a depth of 200 feet or more is 

 reached. These underground waters can percolate through the river-shingle of which 

 the plains are composed with considerable freedom ; thus whenever a hole is dug to the 

 water-bearing stratum, it is quickly filled by the water running in from the shingle all 

 around : the various wells at Eyreton, again, are quickly aflfccted by the state of the river 

 Eyre, which is from one to three or more miles distant from them ; thus if the water in 

 these wells has sunk owing to drought when the river continues dry, it quickly rises 

 again when the river is in flood. Evidently a considerable qviantity of the water of the 

 Eyre and of other rivers flowing across the Canterbury Plains must leak away through the 



